1. Field of the Invention
The present invention relates to a toner for developing an electrostatic charge image, used in electrophotographic image forming apparatus, such as copiers and printers.
2. Description of the Related Art
Recently, energy-saving electrophotographic image forming apparatus, such as copiers and printers, have been demanded because of environmental concerns. An important factor to energy saving is a reduction in fixing energy in particular. To reduce fixing energy, a reduction in the amount of the toner to be applied onto a recording medium (such as paper) has been studied as one of solutions. To reduce the amount of the toner to be applied onto the recording medium, an improvement in the coloring ability of the toner is important.
Examples of the method for improving the coloring ability of the toner include a method of increasing the content of a colorant in toner particles, and a method of improving the dispersibility of a colorant in toner particles. The colorant, which is typically expensive, may increase cost of the toner in the former method. A high content of the colorant in toner particles may increase the influences of the colorant on the chargeability and polarity of the toner, reducing the chargeability of the toner or reducing granulation properties when the toner particles are prepared by a wet method.
In the toner particles prepared by suspension polymerization, a pigment dispersant can temporarily improve the dispersibility of a pigment while this method is susceptible to improvement in stable dispersion of the pigment in a liquid such as a polymerizable monomer.
In preparation of the toner particles by suspension polymerization, a shell layer containing a polar resin is often disposed on the surfaces of the toner particles to enhance stress resistance and chargeability. In such a case, the pigment dispersant may affect the polar resin rather than the pigment in a dispersing step, a granulating step or a reacting step (polymerization step), not attaining a sufficient pigment dispersing effect. The effect of the pigment dispersant on the polar resin may result in an insufficient shell layer disposed, leading to difficulties in precise control of the chargeability of the toner. The effect may also reduce the stress resistance of the toner, preventing high image quality from being kept in long-term use.
Another solution to the reduction in fixing energy is a toner that can be quickly melt at low temperatures to be fixed quickly with small energy. Such a toner has been demanded.
To meet these requirements, the toner has to be softened. Unfortunately, the toner is difficult to simply soften from the viewpoint of heat-resistant storage stability and durability.
Japanese Patent Application Laid-Open Nos. 2002-287426 and 2007-093809 disclose toners the low-temperature fixability of which is improved by incorporating a crystalline resin having sharp-melting properties into toner particles. Through the specification, the term “having sharp-melting properties” means “having a high speed of response to heat.” A known crystalline resin includes crystalline polyester resins.
The present inventors, who have conducted extensive research, found that the dispersibility of the pigment is prone to reduce when a crystalline polyester resin is incorporated into the toner particles. In particular, it was found that the dispersibility of the pigment is prone to reduce when the toner particles are prepared by suspension polymerization.